Shannon Clint Cartee/Notebook/Physics 307L/Lab 00

Lab Title

Experiment 0: Oscilloscope Lab

Lab Assignment, Goals, and Instructions

The Oscilloscope Lab [1]

Experiment 1: The Oscilloscope [2]

Important Dates

Data Collection: 24 August, 2011 and 31 August, 2011

Analysis: 11 September, 2011

Publication: 11 September, 2011

Statement of Purpose

The primary purpose of the Oscilloscope Lab is to facilitate the introduction of students (generally undergraduates undertaking physics or astronomy as their major subject) to modern experimental techniques used by physicists and other scientists. To that end, this experiment will help students gain familiarity with one of the most versatile and prolific instruments available: the oscilloscope. Tertiary goals include students acquainting themselves with proper laboratory safety procedures; learning to setup and use various types of hardware, instrumentation, and software; developing techniques for making and recording precise and accurate measurements; analyzing obtained data using various statistical techniques and error analysis, and then reporting results with respect to statistical and systematic errors [2]. Additionally, collaboration amongst peers and publication of individuals’ results in an open forum is expected.

Materials, Equipment and, Apparatus Used

· Tektronix model TDS 1002 (two channel digital storage oscilloscope) [3]

· B&K Precision® model 4017A (10MHz sweep/function generator) [4]

· Amprobe® model 37XR-A (digital self-ranging multimeter) [5]

· Oscilloscope probe (10X voltage, passive)

· BNC cable (M/M, 50Ω impedance)

· BNC T-Adapter (F/M/F, pass-through)

Safety Considerations

Overall Risk Assessment: Low

Possible Hazards: Electrical shock – equipment attached to mains power (120V, 60Hz, 15A, with ground)

Tripping – power cables and device interconnection cables

Description of Experimental Concepts

· AC (alternating current) coupling

o When an oscilloscope is in this mode, any constant DC (direct current) level coming from the input source is suppressed and only non-zero AC frequencies are displayed [2]. A large capacitor (~ 1MΩ) is connected in series with the input signal forming a high-pass filter, effectively filtering out any
DC-component included in the input signal (the function generator) [6]. Useful when viewing high-bandwidth, non-low-frequency, or low-voltage AC signals that may difficult to see due to superposition of high-voltage DC signal [7].

· DC (direct current) coupling

o When an oscilloscope is in this mode, the high-pass filter is bypassed, allowing the entire input signal to be viewable. The oscilloscope then functions similarly to that of a voltmeter, with input voltage being displayed instantaneously on the display screen, from large positive voltages, through zero, to large negative voltages. Useful when wanting to view the constant voltage in a signal without seeing the
AC-component or when viewing low-frequency signals (~1Hz) [6,7].

· Triggering

o Triggering refers to the manner in which an oscilloscope synchronizes the waveform it displays with the actual input signal. Because an input signal may be at a frequency which exceeds human perception, one aspect of the trigger function is to display only a small sampling of the signal. Furthermore, it may be desirable to begin viewing the signal when it meets a certain criteria such as a threshold voltage level or at a particular point in its cycle. Therefore, the oscilloscope’s display can be configured to trigger when an input waveform increases to a specified voltage (when it has a positive slope – rising) or when a waveform decreases to a specified voltage (when it has a negative slope – falling) [8]. Additionally, by using one of the oscilloscopes auxiliary input connectors, the oscilloscope’s display can be configured to trigger based upon an externally applied signal [2].

· DC Offset

o This control on the function generator controls the amount of DC-voltage which is to be applied its output. The result is a signal that is a waveform superimposed on top of a DC voltage [9].

Relevant Formulas

Equipment Setup

An oscilloscope and function generators were obtained from the storage closet. A digital multimeter was also obtained to serve as an independent device for measuring the output voltage of the function generator. The oscilloscope and function generator were placed on the work bench in close proximity to one another, to facilitate connection with a relatively short BNC cable (~ 1 meter in length) [Osc and FG stacked].

The function generator has a row of buttons for selecting the type of waveform generated (sinusoidal, square, triangle) There are also row of buttons for selecting the order of magnitude of the frequency generated (ranging from 10¹Hz to 10⁷Hz). It has dials which control its output level (voltage amplitude) and DC offset adjustment [FG Right]. There are also dials for fine-tuning the frequency selected on the range buttons (these allow for coarse and fine adjustments). Also present is an LCD display which shows the current output frequency; a button to enable/disable DC offset [FG Left], and three female BNC connectors for output. All other features of the function generator were not utilized in this experiment.

The oscilloscope has many features and capabilities, only a few of which were utilized for this experiment. There are three female BNC connectors for input (two separate channels and one for external triggering). There are dials which control the number of Volts per division, the number of seconds per division, the horizontal and vertical position of the displayed waveform, and the trigger level. There are numerous buttons to switch between channels or to activate menus offer more advanced options [Osc Right]. The oscilloscope’s display screen appears to be made from LEDs (as opposed to older oscilloscopes containing cathode ray tubes). The main screen displays the current active channel(s), inputted waveform(s), the current settings for Volts per division and seconds per division, current cursor positions. When activated by the push buttons, other control menus can be displayed and used to manipulate the functions of the oscilloscope.

All equipment was given a visual inspection to rule out the presence of damage or defects which could pose a safety hazard or confound the experiment; no problems were discovered. The oscilloscope and function generator were then securely connected to mains power and powered on to check for basic functionality; both devices appeared to be in working order [Osc power up screen]; power was then switched off on both devices so that they could be connected.

A BNC T-adapter was connected to the output connector of the function generator [FG Connectors]. This allowed for output signal splitting to accommodate simultaneous viewing of output on the oscilloscope and the multimeter. One end of a BNC cable was attached to one female receiver on the T-adapter; the other end was attached to the CH 1 input connector of the oscilloscope [Osc Connectors]. The remaining female receiver on the T-adapter was used as the test points for the positive and negative probes of the multimeter.

Data Acquisition

In the first part of the experiment, a qualitative examination of the equipment was undertaken. A significant amount of time was devoted to becoming familiar with some of the controls of the function generator and the oscilloscope. For instance, once a preliminary waveform was generated, time was spent adjusting the parameters of the oscilloscope’s display to see the effects manipulating volts/div and sec/div had on the waveform. Conversely, the oscilloscope’s controls were left untouched while adjusting the controls on the function generator, to see what effect was had on the screen’s output.

The next part of the experiment provided an opportunity to put the skills just acquired into practice. The experiment specified to measure the characteristics (period and amplitude) of a sinusoidal wave, using three different methods of collection, using the oscilloscope. First, was to use the grid on the oscilloscope screen and count the divisions between the beginning and end of one period to estimate the waveform’s characteristics. Second, was to use the oscilloscope’s cursor function to make a second estimate. Third was to use and record the output of the oscilloscope’s measure function, which provides these values as determined by the oscilloscope. Furthermore, in measure mode, the waveform’s frequency, V_RMS, rise time, and fall time are also provided.

Four trials were conducted following these procedures, while varying the characteristics of the output of the function generator. All four trials selected a sinusoidal waveform output. The first trial’s parameters were low frequency, moderate voltage, and no DC offset. The second trial’s parameters were high frequency, high voltage, and no DC offset. The third trial’s parameters were low-moderate frequency, low voltage, and no DC offset. The fourth trial’s parameters were low-moderate frequency, low voltage, and a large DC offset.

After these trials were completed and the data for them recorded, a second qualitative phase of the experiment was undertaken, in which the operational limits of the oscilloscope were tested. This was achieved by setting the output of the function generator to its minimum capabilities and observations were made and recorded. Afterwards, the output of the function generator was set to its maximum capabilities and observations were made and recorded.

Analysis, Calculations, and Results

The results of the first part of the experiment are largely qualitative. When increasing the number of volts/div (e.g. 1.00 V/div to 5.00 V/div) the waveform is vertically compressed; when decreasing the number of volts/div (e.g. 5.00 V/div to 1.00 V/div) the waveform is vertically stretched. Similarly, increasing the number of sec/div (e.g. 1.00 μs/div to 2.00 ms/div) the waveform is squeezed horizontally; when decreasing the number of sec/div (e.g. 2.00 ms/div to 1.00 μs/div) the waveform is stretched horizontally. When leaving the controls of the oscilloscope fixed while manipulating the function generator, increasing the output increases the waveform’s amplitude while decreasing the output decreases the waveform’s amplitude. Similarly, increasing the frequency on the function generator corresponds to more cycles of the waveform being displayed on the screen simultaneously, while decreasing the frequency on the function generator corresponds to fewer cycles (or even fractional cycles) of the waveform being displayed on the screen.

The next aspect of the experiment involved a quantitative analysis of various methods (counting divisions, using the cursors, using the measure function) of recording waveform characteristics using the oscilloscope. Table 1 provides a summary of the data collected arranged by Function Generator Output Values, Multimeter Input Values, and Oscilloscope Input Values; the Oscilloscope Input Values are then further subdivided by the three collection methods utilized.

An error analysis was then conducted comparing the divisions method to the measures method followed by comparing the cursors method to the measures method. Although all three sets of data were collected experimentally, thus there is no “correct” value, a qualitative judgment was made to consider the measures method to provide the most representative data set for meeting the criteria of accuracy and precision. Nonetheless, since two experimental quantities are being compared, the percent difference method of error analysis was implemented. In a comparison between divisions counting vs. the measure function the percent difference in amplitude was found to be 3.92%, 0.46%, 1.00%, and 1.26% while frequency was found to be 0.50%, 1.73%, 2.53%, and 1.54% for trials one, two, three, and four respectively. In a comparisons between cursors method and the measure function the percent difference in amplitude was found to be 1.53%, 1.83%, 1.96%, and 0.63% while frequency was found to be 0.51%, 1.22%, 0.16%, and 0.01% for trials one, two, three, and four respectively.

A further error analysis was conducted involving all three methods, in which the mean and standard deviation of the mean were calculated. The results for amplitude were 10.32V_P-P ± 0.17V_P-P, 21.70V_P-P ± 0.15V_P-P, 4.05V_P-P ± 0.04V_P-P, and 3.18V_P-P ± 0.01V_P-P for trials one, two, three, and four respectively; the results for frequency were 100.50Hz ± 0.29Hz, 9,845.58Hz ± 84.54Hz, 393.54Hz ± 3.23Hz, and 388.61Hz ± 2.00Hz for trials one, two, three, and four respectively.

The final aspect of the experiment continued with a qualitative (actually a pseudo-quantitative) examination of the functional limitations of the equipment. The oscilloscope was found to have a horizontal sec/div display range from 5.00ns to 50.0s. When the function generator was tuned to output a square wave at a very high frequency (~10⁶Hz) the image displayed by the oscilloscope became distorted [Dist Img].

Uncertainties in Measurements and Calculations

The primary source of uncertainties in measurements comes from relying on a human data collector correctly reading the oscilloscope’s display then correctly recording the results. Both components of this process contain opportunities for systematic and random experimental errors. Accurately counting divisions (and especially fractions of divisions) on the oscilloscope (which has a small screen) is difficult. Lining up the cursor bars was also difficult to do with great precision. Other sources of random error could have come from electronic interference with the equipment, skewing the displayed values. One method to use to attempt to overcome these limitations would be to rely on the oscilloscopes generated values as provided by the measure function. Another method would be to conduct the experiment away from other electronic equipment. Duplicating the experiment at other times and in other locations would also help in reducing the overall error.

Summary and Conclusions

The lab was successful in its goal in helping the author become comfortable in the experimental lab environment and familiar with several important pieces of equipment. Furthermore, I believe I possess the ability to compile the results and publish them in a thorough and informative manner. However, I was unable to complete portions of the lab assignment due to lack of time and having some difficulty understanding some fundamental concepts regarding the use of the oscilloscope. Furthermore, some difficulty was experienced in publishing the results in a manner which allowed for open access to the results. These difficulties are currently being addressed and hope to be resolved in the near future.

References

1. Koch, S. Physics 307L Junior Laboratory. The University of New Mexico: Department of Physics and Astronomy. http://openwetware.org/wiki/Physics307L:Labs/Oscilloscope (11 September, 2011).

2. Gold, M. Physics 307L: Junior Laboratory Fall 2006. The University of New Mexico: Department of Physics and Astronomy. http://www-hep.phys.unm.edu/~gold/phys307L/manual.pdf (4 September, 2011).

3. Tektronix. Digital Storage Oscilloscopes: TDSS1000B Series. http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=17527&lc=EN (11 September, 2011).

4. B&K Precision Corp. Model 4017A - 10 MHz Sweep Function Generator. http://www.bkprecision.com/products/model/4017A/10-mhz-sweep-function-generator.html (11 September, 2011).

5. Amprobe. 37XR-A True-rms Digital Multimeter. http://www.amprobe.com/amprobe/usen/Multimeters/Compact-Multimeters/37XR-A.htm?PID=73036 (11 September, 2011).

6. Physics Forums. AC/DC Coupling. Physorg.com. http://www.physicsforums.com/showthread.php?t=265365
(11 September, 2011).

7. Alan (aka w2aew). AC/DC Coupling on an Oscilloscope. YouTube. http://www.youtube.com/watch?v=Hkq-fvb5-NI
(11 September, 2011).

8. Ganier, CJ. Using an Oscilloscope. The Connexions Project. http://cnx.org/content/m11902/1.2/ (11 September, 2011).

9. Agilent Technologies, Inc. Adding DC Offsets to a Function Generator’s Output. Agilent Technologies Measurement Tips 6(2), 1 (2009).